1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device. More particularly, the present invention relates to a method of manufacturing a shallow trench isolation.
2. Description of the Related Art
An isolation region is formed in an integrated circuit for the purpose of separating neighboring device regions of a substrate and preventing the carriers from penetrating through the substrate to neighboring devices. In a dynamic random access memory (DRAM) device, for example, the field effect transistors (FETs) are isolated from each other by isolation regions in order to prevent current leakage among the FETs.
Currently, shallow trench isolation (STI) technique employs a method of forming isolation regions. Shallow trench isolation is formed by first anisotropically etching to form a trench in the substrate having a pad oxide layer thereon and a silicon nitride layer on the pad oxide layer, and then depositing an oxide layer in the trench and over the substrate. Next, a chemical-mechanical polishing step is used to planarize the oxide layer to expose the surface of the silicon nitride layer. In the subsequent step, a wet etching step is used to remove successively the silicon nitride layer and the pad oxide layer. At this point, the STI process is finished.
As shown in FIG. 1, since the etching rate of the STI 24 is faster than that of the silicon nitride layer (not shown), portions of the STI 24 removed by the wet etching are thicker than the silicon nitride layer while the silicon nitride layer is removed by the wet etching to expose the surface of the pad oxide layer. Therefore, the STI 24 has substantially the same surface level as the surface of a substrate 10.
However, because the etching rate of the STI edge is faster than that of the centric STI, there are many recess defects 26 formed on the surface of the STI 24 along the top edge of the trench 18 while the silicon nitride layer and the pad oxide layer (not shown) are stripped away. The recess defects 26 lead to leakage and short circuit of the devices. Moreover, the reliability and the yield of the devices are reduced.